1. Field of the Invention
The present invention relates to a laminate for a heat exchanger and more particularly to a laminate comprising a liquid retaining layer for use in evaporative type heat exchangers. The invention also relates to a heat exchanger formed from the laminate and to a method of producing such a heat exchanger.
2. Description of the Related Art
There are a number of situations in which heat exchange in combination with the evaporation of a liquid may be desirable. One such situation is in the humidification of dry air. When air is heated, its ability to carry moisture increases and thus its relative humidity decreases if no further moisture is added. In cold periods, heating installations providing warm air to a building may require some form of humidification in order to compensate for this decrease in relative humidity. In general, relative humidity of less than 50% has been recognised as being undesirable. One way of increasing humidity is to provide water to a porous medium within an air treatment unit. Warm air passing over the medium can pick up additional moisture and transport it into the building. In doing so, heat exchange from the porous medium and its support also takes place. Humidifying devices may be provided independently or may be combined with heaters, heat recovery devices, heat pumps, air conditioners and also with dew-point coolers as described below.
Another situation where heat exchange in combination with evaporation of a liquid is desirable is the evaporative cooler. The principle of evaporative heat exchange has been used for many centuries in various traditional forms. In general, by providing a liquid to one surface of a heat exchange plate and passing a gas e.g. air across the surface, evaporation of the liquid from the surface may take place. The evaporation of the liquid to a vapour requires the addition of considerable heat—namely the latent heat of evaporation. This heat may be supplied by the heat exchange plate and in so doing, will serve to cool it. In the following, although reference will be made to evaporative coolers working with water, air and water vapour, it will be understood that the principles are in general equally applicable to other heat exchange media.
A particular form of evaporative heat exchanger is known as a dew-point cooler. A dew-point cooler attempts to bring down the temperature of a product air stream to as close to the dew point temperature as possible. For air at a given absolute humidity, the dew point is the temperature at which the air reaches a relative humidity of 100%, at which point it is saturated and can absorb no further moisture. The heat is removed from the product air stream by evaporation of a quantity of liquid into another working air stream. Such a process is theoretically extremely efficient and requires no compressor, as is the case for conventional refrigeration cycles. Many attempts have been made to realise such cycles but practical considerations have caused great difficulties in approaching the dew point over most temperature ranges. In the following, the term dew-point cooler will be used to refer to devices which cool a fluid to at or near its initial dew point by heat transfer to cause evaporation of a liquid into a working fluid operating at or near its saturation point
One known form of dew-point cooler operates in counter flow and uses a portion of the product air stream as the working air stream. In simple terms, air flows over a first side of a heat exchange element and is cooled by heat transfer to the element. A portion of the air is diverted back over the second side of the heat exchange element. The second side of the heat exchange element is provided with a supply of water and heat transfer from the heat exchange element to the water causes it to evaporate into the working air stream. Evaporation of water into the working stream requires substantial heat input corresponding to the latent heat of evaporation of water. A device of this type is known from U.S. Pat. No. 4,976,113 A to Gershuni et al. Another device known from U.S. Pat. No. 6,581,402 A to Maisotsenko et al, describes an alternative arrangement of a dew-point cooler in a cross flow configuration. The contents of both of these disclosures are hereby incorporated by reference in their entirety.
It is believed that the supply of liquid to the wet second side of such a cooler is critical in achieving adequate cooling down to close to the dew point. Known coolers have in the past covered the wet side completely with a porous water-absorbent layer. If the air leaving the first side were at the dew point, as it returned across the second wet side, it would initially be unable to take up further moisture since it would already be saturated. It should first be warmed up by thermal input to move away from the saturation line. Only at that point can further moisture be absorbed with a corresponding transfer of latent heat. The presence of a thick porous layer on the wet side however inhibits direct heat transfer from the heat exchange element to the air. For this reason, known coolers rarely descend below the wet bulb temperature of the ambient air. Whilst not wishing to be bound by theory, applicant believes that successful cooling to the dew point can only be achieved in this type of device by providing incremental and repeated alternate thermal heat transfer followed by latent heat transfer. In this way, each time the air absorbs a quantity of water it returns to the saturation line and must be warmed again by direct heat transfer before further water can be absorbed.
It is also believed that to achieve effective cooling, the water activity of the material surface of the wet side must be high whereby it can easily give up its moisture. Water activity is defined by the ratio of the tendency of the material to release water to that of water itself. Thus a surface with a water activity of 1 will easily give up all its water by evaporation into an air flow across the surface while a surface with a water activity of 0 will not release any water under the same circumstances. In the following, reference to water activity is also intended to apply to similar activity of other liquids used instead of water. A smooth metal surface such as aluminium has high water activity and will thus easily give up water. Unfortunately however, it is not good at retaining water and cannot provide an effective buffer of water for evaporation.
It should be noted at this point, that for dew-point coolers, there is an advantage in retaining or buffering water provided to the wet side during periodic irrigations. If the wet side of a dew-point cooler is irrigated, the presence of excess water in the working air stream will cause the temperature to rise from the dew-point to the wet bulb temperature. This is because the excess water causes adiabatic cooling of the working air stream by evaporation of water droplets in the air stream itself rather than by evaporation from the heat exchange wall. Once the irrigated water has been taken up by the surface and any excess has drained away, the temperature can return again to the dew-point. The water taken up by the surface must be sufficient for the dew-point cooler to continue to operate for a period of time until the next irrigation. The ideal liquid retaining layer should thus be able to retain or buffer a large quantity of liquid but should also give it up again easily on evaporation.
A device is known from Dutch patent NL1018735, the content of which is hereby incorporated by reference in its entirety, in which a layer of Portland cement is used to coat the fins of a heat exchanger. Although such a layer has been found to have excellent water activity and water buffering characteristics as a result of its open structure, it nevertheless displays certain disadvantages: it is relatively heavy; it is susceptible to flaking and powdering especially if the carrier layer on which it is formed is subjected to shook or bending; and it is inconvenient to apply in a clean manufacturing environment. In particular, the cement coating must be applied to the formed product, since once coated, the material forming the heat exchanger can no longer be formed. Applying a layer of a desired thickness distribution to a complex shape is difficult and the prior art cement coatings have been found to show undesirable thickness variation.